Electric rectifiers



ug. 7, 955 K. A. MATTHEWS ET AL 29758264 ELECTRIC RECTIFIERS Filed Oct.29, 1952 Inventor K. A. MATTHEWS.

R. A. HYMAN TMA/@0%.

Attorney ELECTRIC RECTIFIERS Kenneth .Albert Matthews and Robert AnthonyHyman, London, England, assgnors to International Standard ElectricCorporation, New York, N. Y.

Application October 29, 1952, Serial No. 317,494

Claims priority, application Great Britain November 26, 1951 7 Claims.(Cl. 317-236) The present invention relates to improvement of electricrectifiers of the kind known as crystal rectifiers.

Crystal rectifiers are of the kind in which a semiconducting crystal ofsuitable material such as silicon or germanium is provided with a baseelectrode making low resistance contact over a relatively large area,and a sharply pointed fine wire electrode or catswhisker makingrectifying contact with its surface. However, as described in thespecification of co-pending British applications Nos. 14429/50 and21040/ 50, the catswhisker may be replaced by a metal film of small areaapplied in a suitable way to the surface of the crystal, to which film alead-out conductor may be soldered or otherwise firmly attached.

One of the chief advantages of crystal rectifiers over those of theselenium or copper oxide type, is the very much smaller capacity betweenthe electrodes which renders such rectifiers suitable for high frequencyapplications, or for cases in which the time constant associated withthe rectifier is required to be small.

However, the forward resistance of crystal rectiers is liable to berather high, and hitherto no satisfactory means has been suggested forreducing the forward resistance without impairing other desirableproperties.

For example, it is known that if low resistivity germanium be used asthe semiconductor, a rectiiier with a few ohms forward resistance andseveral hundred thousand ohms reverse resistance can be produced withoutincreasing the internal capacity above about 1 micromicro-farad, but thereverse resistance characteristic 1s found to be very poor, so that thereverse resistance begins to fall very rapidly when the applied reverseVoltage has reached only a few volts. If the reverse resistance is to bemaintained at a usefully high value for large reverse voltages, highresistivity germanium must be used, and hitherto it has not beenpossible to produce rectiers with very low forward resistance with highresistivity germanium.

The object of the present invention, therefore, is to overcome thisdifficulty, and this object is achieved according to the invention byproviding a rectifying device comprising a semiconducting body having abase electrode making low resistance non-rectifying contact therewith,two rectifying electrodes making contact with the surface of the body,and a single terminal directly connected to both rectifying electrodes,which electrodes are spaced apart by a distance such that therectification ratio States Patent measured between the single terminaland the base electrode is substantially the same as the rectificationratio measured between either rectifying electrode taken by itself, andthe base electrode.

The invention will be described with reference to the accompanyingdrawing, in which:

Fig. 1 shows a circuit diagram used to explain the basis of theinvention;

Fig. 2 shows a graphic diagram illustrating the variation of theinteraction factor between two adjacent rectifying electrodes;

Figs. 3, 4 and 5 show three examples of the manner in which therectifying electrodes may be arranged according to the invention; and

Figs. 6 and 7 respectively show side and end elevations of a rectifieraccording to the invention employing catswhisker electrodes.

In the case of a crystal rectifier, the forward resistance cannot besatisfactorily reduced simply by increasing the area of the rectifyingcontact or electrode, because it will be found that the reverseresistance will be reduced at the same time in a greater ratio, so thatthe rectification ratio (defined as the ratio of the reverse to theforward resistance for the same applied voltage) is reduced as thecontact area is increased. This has been found to be due to a currentinteraction effect which will be explained with reference to Figs. l and2.

Fig. l shows a sectional View of a semi-conducting crystal 1 (ofgermanium, for example) having a base electrode 2 making low resistancecontact with its lower surface, and two similar electrodes 3, 4 makingrectifying Contact with its upper surface. Fig. l is diagrammatic and isnot intended to indicate the actual relative dimensions or arrangementof the parts. In practice, the electrodes 3 and 4 might be thin metalfilms each having an area of the order of l06 square inch, for example.

A direct current source 5 has its centre point connected to the baseelectrode 2. The potential supplied by each half of the source might forexample be 0.5 volt. A switch 6 enables one terminal of a low resistancedirect current measuring instrument or meter 7 to be connected to thepositive or negative terminal of the source S as desired. The otherterminal of the meter 7 is connected through two further switches 8, 9and separate conductors 1t), 11 to the electrodes 3, 4 respectively.

Let it be assumed that the crystal 1 consists of N-type germanium. Ifthe switches 6, 8 and 9 are in the position shown, the meter 7 willregister a current I, given hy E/R1, where E is the potential ofone-half of the source 5 (0.5 volt), and R1 is the forward resistance ofthe rectifier 3 1 (neglecting the resistance of the meter 7). If nowswitch 8 is opened and switch 9 is closed, the meter 7 will register acurrent I2 given by E/Rz where R2 is the forward resistance of therectifier 4-1. Generally l1 and l2 will be approximately equal if theelectrodes 3 and 4 are of the same area.

lf both switches 8 and 9 are closed together, it will be found that themeter 7 now reads a current l which is less than I1, I2, indicating thatthe resistance of the two rectiiiers in parallel is greater than theresistance which would be obtained by connecting two separateresistances R and R in parallel in the normal way. This effect is due tothe interaction of the current flowing through the two rectifyingcontacts, and the magnitude of the effect may conveniently be expressedby means of an interaction factor C defined as The factor C decreasesasymptotically to the value l as the spacing between the electrodes 3and 4 is increased.

If the switch 6 is operated to the lower contact, a similar phenomenonis observed for the reverse currents, except that these currents are ofcourse very much smaller than the forward currents, and furthermore, theinteraction factor C is appreciably less.

It will be evident from this that increasing the area of a singleelectrode will reduce the reverse resistance much more than the forwardresistance of the rectifier, and therefore the forward resistance canonly be reduced at the expense of a reduction of the rectificationratio.

The graph of Fig. 2 shows the relation betwen the interaction factor(ordinates) and the distance (ab- 3 sci's'sa'e) betwee'nth'e 'centres ofelectrodes 3 and 4, which are assumed to be^circular andofdiameter'0100`1'inc`h. Curves 12 and 13 relate respectively to theforward and reverse resistances of the rectifiers. It will be seen thatthe interaction factor for the reverse resistance'pr'actically reachesthe minimum value 'of 1 *so long 'as the separation is greater thanabout 0.004 inch, but for the forward resistance, `the interactionfactor does not substantially reach ythe'v'alue 1 until the 'separationis 0.01 inch or more. corresponding to one of the electrodes 3 or 4without affecting apreciably the rectification ratio, a second similarelectrode may be provided on the surface of -the crystalso long as itscentre is not 'less "than about 0.01 inch from the centre ofthetirstel'ect'rnde, the ltwo electrodes being connected in parallel.Preferably, in order to provide a further `factor of safety, the spacingshould be increased to`p'erhaps002 inch. Nothing appreciable will begained byincreasingthe'spacing further than this.

The curves shown in Fig. 2 are substantially independent of the area ofthe electrodes '3 and 4, but will be different for diitereritsemi-'conducting materials. It will beunderstood that Fig. 2 appliestohigh resistivity N- type germanium.

This 'principle 'may be extended -by providing any number of electrodessimilar to -3 and 4 on the surface of the same crystal and by connectingthem all directly in'parallel to a common terminal, so long as no twoelectrodes have their centres closer than about 0.02 inch.

The composite r'ecti'er Vso formed will lthen have .a forward resistanceapproximately equal to l/nth of the formed resistance of a rectier witha single electrode, where n is the number of electrodes, but therectification ratio will be substantially the same as the averagerectification ratio for all the electrodes taken singly.

The rectifying electrodes maybe arranged, for example, in line, or insquare or hexagonal patterns, `as shown inligs. 3, 4 and 5 respectively,the distance between the centres of any two adjacent electrodes beingnot less than 0.02 inch, Vfor example. As indicated in Fig. 3, all therectifying electrodes V14 should be connected by suitable wires 15 to acommon terminal 16. Similar connections should be used in Figs. 4 `and 5but they have not been shown in order to avoidcomplicating the iigures.

lt will be understood that the spacing chosen depends on therequirements. Possibly a small reduction of the rectification ratio(such as 1% 'for example) might be allowed, in which case the spacingcan be reduced so'that the value of C as rea'd from curve 12 does .notexceed 1.01, for example. Since for spacings greater than. about 0.005the interaction for the reverse resistance is negligible, as shown bycurvei13, it is only necessary to consider the interaction for theforward resistance.

Although the rectifying electrodes have so far been assumed to bemetaliilms of small area, substantially similar interaction effectsoccur with` catswhisker electrodes, and `the'forward resistance can bereduced in the same way without reducing the rectication 'ratio byproviding a brush of catswhiskers arranged in contact with the crystalsurface so that two-adjacent contact points are not closer than, say,0.02 inch. An arrangement of this kind is shown in Figs. 6 and 7. Thesemiconducting crystal 1 with the usual base electrode2 isprovided withany number of tine wire catswhiskers 17 held in a metal strip I8 whichforms the common-terminal .for all the catswhislrers. The catswhiskersshould preferably be bent into an VS shape, as show-n-inflig. =7,inorder-to provide suitable springpressure on the surface of thecrystal.

It will be understood that in all=cases the surface of thesemiconducting crystal should betreated inthe conventional way in ordertoobtaingood rectiicationrproperties, and, generally, the device will bedesignedand constructed accordingto well known practice, except as Thusin order to ha'lve the v'forward resistance regards the provision ofadditional electrodes arranged 'and spaced'intlre'manner exptained.Thegures ofthe drawing are not intended to indicate constructionaldetails.

While best results have .been obtained with N-typc germanium as thesemiconducting material, it will be understood that `Ptype germaniumI'could also be used, or silicon, or other known .,rectifyingcrystals.

It will be evident, of course, that if-tlrerectiier has n electrodesconnected together in the manner explained, the capacity of thecomposite rectier will be approximately 'n times the capacity `obtainedwith one of the electrodes measured singly.

Attention is directedto ABi'itishpatent specification No. 654,909 inwhich a rectifying crystal is illustrated with two or morecatswhis'ker'el'ectrodes arranged in a line or circle. This arrangementdiiers from the rectitiers of the, present inventionin this, that itcomprises-a series of separate rectiliers separately employed, andaccordingly, the catswhiskers cannot be .connected .in parallel, buteachhas a corresponding load impedance connected to it. Furthermore, thespacing of .the electrodes must be such that there is some-.currentinteraction between adjacent electrodes to permit successive primingof`the trigger circuits associated with the electrodes vas described. Inthe case ofthe presentinvention, thisinteraction is to be avoided andthespacing .of the electrodes will therefore be -such that the devicewould not work in the manner described in specication No. 654,909 if thelelectrodes were .disconnected from one another.

While the principles of the invention have been described .above inconnection with specific .embodiments and particular modicationsthereof, it -is `to be clearly understood that this=description is madeby `way of example .andnotas .a limitation'on the scope of theinvention.

What we claimis:

1.A .rectifying device .comprising a semi-.conducting body having abaseelectrodemaking low resistance, nourectifying .contact therewith,two .rectifying velectrodes making contact.with the surfaceof.tlie`body,.anda single terminal directly connected to bothrectifyingelectrodes, which electrodes arespacedsapartbya distancewhereby the rectification ratio measured between the .single terminaland-the base .electrode'is substantially the same as the rectificationyratio measured ,between either rectifying electrode taken .by itself,and the vbase electrode.

2. A rectifying device .comprising a body of semi-conducting materialhaving a base electrode makinglo-w resistance non-rectifying Contact'thereu'fi'tln and two rectifying electrodes directly connected togetherand making rectitying contact with the surface of the body, saidrectifying'electrodes being spaced apart "by a given distance wherebythe interaction'factor correspondingtto forward currents is:substantially equal to l.

3. A rectifying device comprising .a semi-condncting crystal" having abaseelectrode making low resistance nonrectiyingcontact therewith, anda.plurality of rectifying electrodes making rec'tifying Contact with thesurface of the crystal, the said rectifying electrodes'being directlyconnecte'd'together, and alLpairs-of adjacent rectify/ing electrodesbingspaced apart' by a 'distance not'less than .02 inch whereby `theinteraction of forward currents passing respectively through theadjacent-electrod-es is negligible.

,4. A `rectify-ing .device V.comprising'a semi-conducting crystal ofN-typegermaniu-m having-a base electrode mal-:- ing low resistancenon-rectifyingContact;therewith, apluralityf of :similar rectifying#electrodes finaking rectiiying Contact with the surfaceofhe crystal,:each rectifying electrode :comprising ai thincirclar. .metalliciilmofidiameter about 0.001 inch, and means for directlyconnecting eachurectiiying:electrodeI toza. common terminal, 1 fthe: saidrectifyingzelectrodesfbeing `spaced apa'rtrby a distance of not lessthan 0,02 inch.

5. A rectifying device according to claim 3 in which References Cited inthe file of this patent the rectifying electrodes comprise sharplypointed wires l UNITED STATES PATENTS glrellameally and eleetrleallyheld together t0 form a 21524103 5 Bardem et al. Oct 3 1950 ystal 1natleast one stralght line. 7 80 o electrodes are arranged in contactwith the surface of the crystal in a plurality of intersecting linesforming a pattern l0 of squares.

1. A RECTIFYING DEVICE COMPRISING A SEMI-CONDUCTING BODY HAVING A BASEELECTRODE MAKING LOW RESISTANCE, NONRECTIFYING CONTACT THEREWITH, TWORECTIFYING ELECTRODES MAKING CONTACT WITH THE SURFACE OF THE BODY, AND ASINGLE TERMINAL DIRECTLY CONNECTED TO BOTH RECTIFYING ELECTRODES, WHICHELECTRODES ARE SPACED APART BY A DISTANCE WHEREBY THE RECTIFICATIONRATIO MEASURED BETWEEN THE SINGLE TERMINAL AND THE BASE ELECTRODE ISSUBSTANTIALLY THE SAME AS THE RECTIFICATION RATIO MEASURED BETWEENEITHER RECTIFYING ELECTRODE TAKEN BY ITSELF, AND THE BASE ELECTRODE.